Hurricane Harvey is the first major hurricane to make landfall in the United States since Wilma in 2005, ending a record 12-year period with no storms making landfall as a major hurricane. Driven by relentless rains from Tropical Storm Harvey, the flood event becomes one of the worst disasters in modern U.S. history, which unfolded on Sunday, August 27 in the Houston area. Rainband after rainband swept north and translated slowly east through the metro area from Saturday into Sunday morning, dropping totals on the order of 20”-25” of rain. Freeways and streets throughout the Houston area were impassable on Sunday morning, and many hundreds of homes were flooded, with numerous reports of trapped residents. At least 6 deaths had been reported in the Houston area by end of Sunday. Throughout Texas, more than 300,000 people were left without electricity.
Rainfall Total and Record High Storm Surge by Sunday, August 27, 2017
As of 10 am CDT Sunday, the National Weather Service Storm Summary (updated every 6 hours) listed four locations near Houston that had received over 24 inches of rain since the storm began on Thursday. By far the two-day rainfall around Houston is now near or above 20 inches nearly everywhere, including a peak amount of 27.45 inches or 697 mm near Dayton. Another 6-15 inches is forecast on Sunday night, mostly to the north of the city. Below is a map showing the spatial distribution of the rainfall map. In the Greater Houston/Southeast Texas region, an estimated 9 trillion gallons (3.4×1010 m3) of rain had fallen by the afternoon of August 27, more than any documented tropical system in U.S. history. The rainfall rate of 2.88 inches per hour was recorded at Buffalo Bayou, TX on Sunday as well.
At 1:24 pm CDT Sunday, the Manchester tide gauge, located on an estuary on the east side of Houston, recorded a remarkable storm surge that was nine feet and rising. This surge was likely 95% due to the huge amounts of freshwater river runoff trying to drain into Galveston Bay from August 26 morning’s torrential rains. This runoff was unable to drain because of strong onshore winds that were pushing a salt-water storm surge inland, opposing the freshwater runoff. The storm tide (combination of the surge plus the tide) was 8.66’ above the high tide mark (Mean Higher High Water, or MHHW) at Manchester at 1:24 pm CDT. This is a new record-high water level for the gauge, going back to when records began in 1998.
The rainfall totals over the past 72 hours are astounding. Harris County has recorded more than 32 inches of rainfall; Galveston County has registered just shy of three feet, at 34 inches. For Monday, Houston probably got a bit of a break from the heaviest downpours. The National Weather Service calls for “light to moderate rains across the area today and tonight” as Tropical Storm Harvey moves briefly offshore. Hopefully, this will allow some of the floodwaters to drain away in Houston, and if we’re really lucky there will be a break between the bands of additional rainfall that are currently projected to arrive on Tuesday and Wednesday in eastern Texas and southwestern Louisiana. According to NWS Houston, Monday's forecast rain from 7 AM to 7 PM is expected to be relatively lighter, up to 5" additional, isolated more.
Our top models have become a bit more consistent on Harvey’s short-term future. The storm’s center is now expected to move slightly offshore by Monday, still as a tropical storm, then return onshore by Wednesday on a northward track expected to be near or perhaps just west of Houston. The updated Harvey track map from NHC at 10am shows tropical storm force winds possible through Wednesday with still heavy flooding.
Based on NWS’s Weather Prediction Center’s 5-day river flooding forecast, a list of zip codes in TX that are currently impacted by flood induced by Harvey were created by JLT (orange area only). The region of impact is downloaded from NOAA WPC website, with event date from August 27 to Sept 1. The purpose of creating the list of impact zip code is to allow JLT Specialty to call their clients about these locations start from Monday morning. Below are the sample results for the impacted area in Houston, TX.
ESRI published the estimated Harvey flood footprints for the next 5 days (August 28 to Sept 1) for the State of TX. A screenshot can be found below for Houston. This is based on 2D flood simulation with forecasted rainfall on the digital terrain. Red color represents deeper water, while green color means shallow water. No depth information is given at this moment.
The rains in Cedar Bayou, near Mont Belvieu, Texas, reached 51.88 inches (132 centimeters) as of 3:40 p.m. CDT today. That's a record for both Texas and the continental United States, but it does not quite surpass the 52 inches (133 centimeters) from Tropical Cyclone Hiki in Kauai, Hawaii, in 1950 (before Hawaii became a state). The previous record was 48 inches set in 1978 in Medina, Texas, by Tropical Storm Amelia. A weather station southeast of Houston reported 49.32 inches of rain.
Rain totals for Tuesday afternoon and tonight showed that most of the area expecting less than an additional 6".
During noon time of Tuesday, FEMA published the Harvey flood depth grid for Harris County. This flood footprint was created based on a 5m DEM obtained via USGS (NextView data). The stream/river gages data were obtained from the National Weather Service (http://water.weather.gov/ahps/download.php). Gage readings will be updated as streams to crest. Gages that showed signs of malfunction or stopped reporting were not utilized. Quality control is included at every step in the process, which include examining flood extents polygons and comparing to ground elevation data to identify anomalies that may suggest a source data problem (areas of more widespread flooding or areas where flooding seems unusually small). Delivered geo-databases include the following layers:
- Depth grid
- Approximate flood extents
- Gage points used (includes stage readings, “gage 0” value, and calculated water surface elevation.
- Areas of uncertainty
FEMA also performed flood damage assessment in the impacted area within TX. Below is the preliminary summary table showing the total number of structures and the affect level (depth of flood) for each county by Harvey in the state of TX. Of the note, this does not account for elevated structures and assumed that the parcel centroid is the building location. In addition, this does not account for wind damage, flooding from dam or levee breaks, flooding as a result of failure of irrigation ditches, use of spillways and weirs, small tributary flooding with minimal or no gage data, or stormwater backup. This data will be refined by FEMA using event imagery, when provided (yellow highlighted cells indicate damage assessment categories that need to be further reviewed due to discrepancies with the modeled output results).
This map shows the FEMA created flood depth grid for Harris County, and the structure locations of flood damage assessment.
Based on FEMA created flood footprint for Harris County, JLT performed the analysis and identified Harvey impacted portfolio locations of UFG (221) and Church Insurance (2). According to FEMA’s preliminary Damage Assessment table (number of structures) in the previous page, since Harris County has the most flood loss, almost accounting for 70% of total loss, this analysis provides insights for our client for the majorities of their potential loss.
As for Harvey’s flood footprints, besides ESRI data and KatRisk predicted inland flooding areas, we also found Pacific Northwest National Laboratory also published daily forecast precipitation and forecasted flood footprint until August 29. The simulation used a 2-Day hindcast and a 5-Day forecast using the QPF data. Two simulation domains were utilized, one of which covers watershed within the heavier rain bands. A post-processing algorithm (CA) was used to refine the spatial resolution from the 2D hydrodynamic 120-meter simulations, with a focus on the Houston metropolitan area. The refinement is initialized from the coarse hydrodynamic simulation and uses hydrostatics to refine flood extents using 10-meter elevation data. The web link can be found at:
As tropical storm Harvey moves east, the weather forecast will improve for Houston for tomorrow. Harvey will spend much of Wednesday dropping rain on Louisiana before moving on to Arkansas, Tennessee and parts of Missouri, which could also see flooding.
More Rain Still to Come for Louisiana
Even more concerning is the prospect of additional extreme rains—one to two feet or more—before Harvey moves out of southeast Texas later this week. Points east from Houston into southern Louisiana will also be at an increasing risk of torrential rain and major flooding over the next several days. Harvey’s circulation is located in a near-ideal spot for funneling vast amounts of moisture from the Gulf of Mexico toward the upper Texas coast. Here, converging winds at low levels have been concentrating the moisture into north-south-oriented bands of intense thunderstorms with torrential rain. Since Harvey is barely moving, these bands are creeping only slowly eastward as individual cells race north along them—a “training” set-up that is common in major flood events.
Mesoscale models, our best guidance for short-term, small-scale behavior of thunderstorms, show little sign of relief for southeast Texas anytime soon. Convection-resolving mesoscale models, which have a tight enough resolution to depict individual thunderstorms, are an invaluable tool in situations like this. The mesoscale nested NAM model predicts that ~20” of additional rainfall is likely through Friday across the Houston metro area, with even larger totals at some points, as indicated by the figure below.
The threat of flash flooding is increasing across Southern Louisiana as soils saturate, while significant to catastrophic flash flooding continues across portions of Southeast TX, including the Houston metropolitan rea. The average rainfall within the Harris County Emergency Management Network has exceeded that of Tropical Storm Allison (2001) in almost half of the time (2-3 versus 5 days). Historic flooding is expected to continue in the Houston metropolitan area through the foreseeable future. Given that this event still has 2 to 4 more days to go, it appears likely that some location in Texas will break the all-time record for U.S. rainfall from a tropical cyclone or its remnants: 48.00” in 1979 from Amelia. Another record to watch: the all-time 24-hour U.S. record rainfall of 42", set in Alvin, TX (just south of Houston) on July 25-26, 1979, in association with Tropical Depression Claudette.
As for the estimate of hurricane induced inland flood loss, Tropical Storm Allison caused $9 billion (in 2001 USD, equal to $12 billion USD as of 2017, of which $5 billion USD was insured) in damage and 41 deaths in June 2001. Given the fact that the rainfall intensity and its duration of another 2-4 days, with an estimated additional 5 to 10 trillion gallons (1.9×1010 to 3.8×1010 m3) of rain are expected before the storm dissipates, the flood loss caused by Harvey would be much worse than Tropical Storm Allison mentioned above.
